During conventional combustion processes, such as coal gasification processes, waste gas streams containing hydrogen sulfide, carbon dioxide and carbon monoxide are generated. Typically, before such a gas stream can be safely discharged to the environment, it is desirable to substantially remove at least the hydrogen sulfide component therefrom.
A variety of processes have been developed for the scrubbing of hydrogen sulfide from waste gas streams. A particularly well-known process is a scrubbing method using a reactive solution, such as the one generally referred to as a Stretford solution. For this process, the waste gas, typically at a temperature of about ambient to 170.degree. F. (20-77.degree. C.), is discharged into an open venturi mixer or scrubber. An alkaline oxidizing solution, for example a Stretford solution, is simultaneously injected into the venturi scrubber, typically at a temperature of ambient to about 120.degree. F. (49.degree. C.). Through absorbtion into, and chemical reaction with, the reactive solution, the hydrogen sulfide generates solid, elemental, sulfur, or other sulfur-containing substances, which can precipitate. A typical Stretford solution is an alkaline solution of sodium carbonate or sodium bicarbonate, containing a Vanadium V oxidation/reduction agent, and anthraquinone disulfonic acid disodium salts. During the chemical reactions, the Vanadium V is reduced to Vanadium IV. It is noted that a variety of alternatives to the Stretford process are well-known. Generally, variations involve substitution of different oxidizing solutions for the Stretford solution. Such process include: the Klaus process; the Sulfolin process; and, the Hyperion process.
Even following a conventional venturi scrubber process using Stretford (or similar) solution, the off gases typically still contain a significant amount of hydrogen sulfide therein, generally on the order of less that 100 to several hundred ppm to above 1000 ppm. Reasons for this may relate, in part, to the fact that in a typical non-packed (i.e. open) venturi scrubber, mixing, i.e., interphasing of the gases and the reacting liquid solution, even when optimized for high yield and preferred flow rate, still is not sufficient to provide 100% or near 100% reaction.
To at least partially accommodate this, the off gases from a typical venturi scrubber are often passed through a second scrubber or absorber. Typically, the second absorber is a packed bed absorber, with counter-current liquid and gas flows. That is, the waste gases, including some hydrogen sulfide therein, are bubbled into the packed bed absorber from one direction, and the reactive oxidizing solution (for example a Stretford solution) is directed through the absorber from an opposite direction. The packing in the absorber generally acts to increase intimate contact of the gas and liquid phases, and thus facilitates reaction.
For a typical industrial process, the packing material comprises pieces of materials such as metals, plastics, ceramics, wood, etc. A typical packed bed scrubber or absorber, for many commercial processes, comprises a very large column, for example a column on the order of about 20 ft. high, and 3 ft. or so in diameter.
A source of continual problems for industries utilizing combustion processes has been the fact that packed bed scrubbers readily plug during scrubbing processes. That is, elemental sulfur, sodium sulfates and other solid materials form during reactions in the packed bed scrubber. Generally, the solids form at the packing surface, and relatively rapidly plug the absorber, to inhibit the flow of fluids therethrough. Thus, in some instances even after only a few days of use, a packed bed scrubber must be taken off line and reconditioned or recharged.
Typical methods of reconditioning packed columns include: replacement of the packing material; or, a cleansing of the packing material in a manner generally dissolving the plugging material, for example upon aggressive washing with a strong, hot, highly caustic solution such as a sodium hydroxide solution or the like.
The problem of relatively rapidly plugging packed absorbers is significant. First, the off time for the absorber can be substantial, inconvenient, and/or in violation of governmental permits. Back pressures can be inconvenient or hazardous, if not appropriately controlled. Further, commonly used, typically highly caustic, cleansing solutions may limit packing life and may otherwise damage, in time, the absorber unit involved. Further, large amounts of highly caustic washing solution create a substantial cost, and also a waste disposal problem.
Replacement of packing is also undesirable. It is, for example, time consuming, expensive, potentially hazardous and inconvenient. Also, frequent disturbance of bed packing can inhibit good column operation.
Those involved in industries using combustion processes have attempted to alleviate the column-plugging problem by trying various sizes and shapes of packing material. For more exotic-shaped packing materials, plastics have been used, since plastics are readily formable into a variety of shapes, at a relatively low cost. In general, approaches to the above-related packed scrubber problem, involving use of various packing shapes, have been either ineffective, or not of substantial benefit in leading to a significant alleviation of the problems.
What has been needed has been a method applicable to yield a packed bed scrubber unit, with a reduced tendency toward plugging during operation. Packed bed arrangements facilitating reduced plugging and regeneration of packing have also been needed.